1. Field of the Invention
The present invention relates to a process for producing a thermoplastic resin by means of emulsion polymerization using a conjugated diene rubber, an aromatic vinyl compound and a vinyl cyanide compound, followed by flocculation and subsequent direct pelletizing. More particularly, the invention relates to a process for producing a thermoplastic resin having improved impact resistance by adopting the direct pelletizing to omit a drying step which causes oxidation deterioration of the resin, and further by effecting the direct pelletizing in the presence of a particular phenolic compound to prevent the resin from thermal deterioration in the absence of oxygen.
2. Description of Related Art
In general, ABS resins are known to be graft polymers prepared by polymerizing an aromatic vinyl compound and a vinyl cyanide compound in the presence of a conjugated diene rubber or to be the mixtures of such graft polymer further blended with a copolymer of an aromatic vinyl compound and a vinyl cyanide compound. Because they have excellent properties in processability, mechanical strength, gloss, chemical resistance and so forth, they are widely used in many fields today. In recent years, for the purpose of imparting heat resistance to the resins, there have been some attempts for improvements such that a part of the aromatic vinyl compound is replaced with an unsaturated carboxylic acid alkyl ester compound or an imide compound.
The graft polymers are known to be produced by emulsion polymerization, bulk polymerization, suspension polymerization, solution polymerization or the like, but the emulsion polymerization is widely adopted because of its contribution to the properties of the resin such as gloss and impact strength of the products, its safety in the process and so forth. The graft polymers obtained by the emulsion polymerization is, if necessary after being mixed with a copolymer of an aromatic vinyl compound and a vinyl cyanide compound, usually subjected to the steps of flocculation, solid-liquid separation and hot air drying to make powdery products.
However, this process has a problem in that the impact resistance which is characteristic of the ABS resins is not sufficiently revealed. This is because the graft polymers or the optionally added copolymers undergo oxidation deterioration during the hot air drying. In order to stabilize the resins during the hot air drying, there have heretofore been applied such methods as incorporating a phenolic antioxidant of various kinds, or incorporating one or more other antioxidants including sulfur-containing antioxidants, phosphorus-containing antioxidants and amine antioxidants in addition to the phenolic antioxidant.
For example, the phenolic antioxidants conventionally applied are n-octadecyl 3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate, 4,4'-butylidenebis(3-methyl-6-t-butylphenol), 4,4'-thiobis(3-methyl-6-t-butylphenol), 2,2'-methylenebis(4-methyl-6-t-butylphenol), a formalin condensation product of nonylated para-cresol, 2,6-di-t-butyl-4-methylphenyl and the like. The sulfur-containing antioxidants conventionally applied are pentaerythrityl tetrakis(3-laurylthiopropionate), dilauryl 3,3'-thiodipropionate, dimyristyl 3,3'-thiodipropionate, distearyl 3,3'-thiodipropionate, lauryl stearyl 3,3'-thiodipropionate and the like. The phosphorus-containing antioxidants conventionally applied are tris(nonylphenyl) phosphite and the like.
Although the application of such antioxidants considerably improves the stability of the resins during the hot air drying, it has still some problems in that the ABS resins after drying become discolored, and further in that the resins is not necessarily satisfactory for producing shaped articles of high impact resistance. As a method for resolving such problems, there has been proposed a direct pelletizing process which is conducted after flocculation by eliminating the hot air drying step which causes severe oxidation deterioration to the resin. However, since the aforementioned phenolic antioxidants, phosphorus-containing antioxidants, sulfur-containing antioxidants, amine antioxidants or the like are scarcely effective to the thermal deterioration which occurs in the absence of oxygen during the direct pelletizing, for example, inside an extruder, the direct pelletizing still has a problem in that it hardly controls the failure of the impact resistance.